Chen Xiao Fan wrote:
> Maybe FFT is the way to go for PF analyzing.

FFT would not be very useful.  You are looking for relative phase
relationship between two signals, not the spectra of the signals separately.

Think of the case where the voltage is a pure sine and you have three
different loads, resistive, capacitive, and inductive.  If the magnitude of
the load currents are the same, an FFT will give the same result in all
cases even though one has a power factor of 1 and the other two of 0.

To compute power factor, compute the RMS voltage and current, and the actual
power.  Power factor is the ratio of the true power divided by the product
of the RMS voltage and current.  You can see that this can be at most 1,
which is what you get only with a resistive load.  The extremes are pure
capacitive and inductive loads where the delivered power is 0 and the power
factor therefore is 0.  Technically an active load could produce negative
power factors, but this means the load is actually delivering power back to
the AC line.


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